There are disadvantages associated with the commercial exploitation of the continuous preparation of iso- or terephthalic acid by the catalytic air oxidation of m- or p-xylene in the presence of liquid benzoic acid as solvent at operating temperatures in the range of 170.degree. to 274.degree. C. and operating gauge pressures in the range of 21 to 35 kg/cm.sup.2 and the requirement to remove by-product water as it is formed according to the disclosure of British Patent No. 1,088,183 published Oct. 25, 1963. The disadvantages are highly discolored to charred product and, under the required conditions for operating pressure and removal of by-product as it forms to use a near zero water content benzoic acid solvent, widely varying oxidation zone temperature which would be unacceptable for commercial operation.
For example, at start-up of such continuous oxidation at a gauge pressure of from 21-35 kg/cm.sup.2 and removing by-product water from 5 weight percent down to zero in the benzoic acid solvent the oxidation zone temperature, we found, can increase substantially instantaneously as much as about 110.degree. C. without causing an increase in the operating pressure of 21 to 35 kg/cm.sup.2. Such a drastic increase in oxidation zone temperature causes charring of product, intermediates and/or solvent.
We also found that changes in water content of the benzoic acid solvent during continuous operation with by-product water removal after successfully starting without the drastic temperature increase leading to said charring resulted in cycling of oxidation zone temperature above and below a desired constant temperature at a selected constant gauge pressure.
Copending patent application Ser. No. 708,358, filed July 26, 1976, discloses the preparation of iso- or terephthalic acid by the catalytic air oxidation of m- or p-xylene in the presence of liquid benzoic acid solvent which permits commercially feasible temperature control in the range of 175.degree.-235.degree. C., provided the oxidation is conducted in a stirred reaction zone; the solvent consists essentially, on a weight basis of 85 to 97% benzoic acid and 15 to 3% water; and the oxidation zone gauge pressure is constant within the range of 7 to 21 kg/cm.sup.2. Said method of iso- or terephthalic acid production is applicable for weight ratios of the benzoic acid-water solvent system to xylene in the range of 2:1 to 8:1. The xylene conversion is substantially complete and high mole percent (% of theory) yields of iso- or terephthalic acid are produced at air ratio which provides 3-10 volume percent oxygen in the exhaust (water and benzoic acid free basis) from the oxidation zone. The catalysis for such oxidation can be provided by manganese, cobalt, or manganese and cobalt as transition metal oxidation catalyst in combination with a source of bromine.
The continuous conduct of such temperature controllable process for preparation of iso- or terephthalic acid comprises simultaneously charging air, xylene and liquid benzoic acid-water solvent system containing the components of catalysis to a stirred tank-type oxidation vessel having a means for withdrawing the fluid oxidation effluent (suspension of crystalline phthalic acid product in the liquid solvent system, means for escape of exhaust (vapors of benzoic acid and water) and the gases, nitrogen, oxygen and oxides of carbon), and means for returning liquid benzoic acid-water condensed from said exhaust in an amount to maintain the 85-97% benzoic acid and 15-3% water solvent system in the oxidation zone. Fluctuations of the oxidation zone temperature are minimized by adjustment of the rate of such benzoic acid-water condensate returned to the oxidation zone. That is, the water content is increased as said temperature increases or decreases as said temperature decreases from a selected operating temperature. The water content of the solvent system in the oxidation zone cannot be permitted to go below 3 weight percent because unacceptably high temperature will occur in the oxidation zone or to go above about 18 weight percent because the xylene oxidation is at a negligible and commercially unacceptable rate.
The present invention provides a novel system for preparing an oxidation charge stock of xylene, solvent system and components of catalysis as well as a novel means for accomplishing control of oxidation at substantially constant temperature without fluctuation of more than .+-.5.degree. C., preferably .+-.1.degree. C., therefrom.